HIGHER CRUSTACEANS (MALACOSTRACA) 829 



and logs, in holes, or among vegetation. But they are present 

 practically everywhere, and in most bodies of water, even small 

 ones, one or several forms may be expected to occur. Certain 

 forms (burrowing crayfishes) do not live in open water, but burrow 

 in the ground, going down to the ground-water; their presence is 

 indicated by piles of mud, brought out of the holes. 



Fresh-water Malacostraca are found, with exception of the An- 

 tarctic regions, practically all over the world, including the Arctic, 

 but naturally are most abundant in the tropics. A number of 

 groups are distinctly characteristic of temperate climates, and at 

 least one group (genus Cambarus, crayfish) has reached its highest 

 development in North America. Here Malacostraca are found 

 everywhere, but chiefly in the interior basin with its great an4 

 diversified river systems. They become rather scarce on the west- 

 ern plains and in the arid regions, but are not entirely missing 

 there. The various forms are adapted to different surroundings; 

 some prefer large rivers, others creeks or ponds, or small pools, 

 springs, and even subterranean waters. 



They belong to very different groups of the subclass Malacos- 

 traca. The latter has been divided, in the more recent systems, 

 into ten orders, and of these four possess representatives in our 

 fresh waters: Isopoda, Amphipoda, Mysidacea, and Decapoda. 

 These differ very much in their outer features, in general shape 

 of body, size, color, and details of morphology, so that it is hard 

 to give a short general account of their characters. 



The body may be only a few millimeters long, up to one or two 

 centimeters (Isopods, Amphipods), or it may be somewhat longer 

 (Mysidacea and some Decapods), while in other cases (prawns and 

 crayfishes among the Decapods) it may reach the considerable 

 length of ten centimeters and over. In the smaller forms, the 

 color is generally inconspicuous, whitish or grayish, often more or 

 less transparent. The larger forms have more distinct colors, 

 which may become quite brilliant in certain parts of the body: the 

 large claws of the genus Palaemon (prawns) are, in the male sex, 

 often red, blue or purple. The crayfishes are, in general, of green- 

 ish or brownish olive tints, but as a rule adult males are more 

 vividly colored, and in some species the adult male assumes a color 



